Electricity is LETHAL! The following information is provided for those who are knowledgeable, experienced and comfortable working with and around 240 volt ac electrical systems and high temperature ceramic kilns. If you are not, please do not attempt to implement any of the following suggestions without the help and assistance of a licensed electrician.
In addition to the electrical dangers, electrical and firing problems can easily lead to accidental fires that can destroy structures not to mention the possibility of killing or injuring people. While the author has executed the following modifications and those modifications are currently in use in a full time pottery studio at the time of this writing, the author does not accept any responsibility for any injury, damage or other losses that may result from attempting to implement any of the suggestions and designs shown in the following page. The use and implementation of these suggestions are at your own risk.
Knowing a little about electrical theory will remove some of the mystery from the wiring and may save some expensive visits from the electrician. Electricians and the people who design kilns are not super-people. They learned their craft the same way that you have learned to manipulate clay, by trial and error and mistakes along the way. Don't try to remember everything in this article or other texts, but rather use them as references to return to when you run into a problem. This description is limited to 110 to 220 volt systems although the principles used with 440 volts are similar.
The general way that electricity is delivered to the studio is through three wires running from the transformer to the building. Two of these wires are insulated and the third is usually bare. The two insulated wires are 110-volt lines that are identical in every respect except for their timing.
For convenience, the two insulated lines are called "A" and "B." Either of these lines can be wired with the neutral line to provide electrical power at 110 volts. In any building, the wiring is divided about half and half to distribute the load equally between the two incoming lines. As Figure 3 shows, each line cycles back and forth from plus 110 volts to minus 110 volts.
The only difference between the two is that when line "A" is headed
up toward plus 110 volts, line "B" is headed down toward minus 110 volts.
As either line
peaks, the other line is bottoming out and the two are 220 volts apart, so while a voltmeter testing either line will show only 110 volts, it will read 220 volts when connected to both the "A" and "B" lines. (See Figure 4.)
The third wire is neutral or ground. Although a current of electricity flows through it, its voltage is zero with respect to the earth, and hence the name "ground." Although it is normally safe to come in contact with this line, there are malfunctions that could make it lethal. Within the building and appliances, the fourth wire (normally green or bare) is used as a safety ground. Most shorts or leakages will take this path rather than through the human body.
Comparing electrical wires with plumbing pipes works as a good analogy in most, but not all, cases. Voltage is the pressure forcing the electrical current through the wire. Amperage is the measure of the speed of flow through the wire, while watts would be the measure equivalent to gallons per minute. The neutral line is the equivalent of the drain pipe.
Just as a larger pipe can carry more water at the same pressure, a larger wire can carry more electrical current, with a big exception. If a pipe is too small, the only negative result is that less than the necessary amount of water will trickle through. With wiring, if the wire is too small, it begins to function the same way the heating element does and gets hot. This heat can quickly build up enough to melt the insulation, set fires, and electrify objects including people and pets.
Using 220 volts instead of 110 is more efficient. Since l the voltage (pressure) is twice
as high, twice as many watts can be carried through the same wire without overloading it. The cost of the wire alone in a 220 volt circuit will be cut in half. The losses due to heating in the supply wires are also reduced, and finally the balance of electrical supply to the building is maintained.
Standard Electrical Signposts
There are standards in the electrical industry that will help you avoid some basic wiring mistakes, stay in compliance with codes and help the next person who has to work with your wiring. These are for 110/220 volt power circuits.
Electrical plugs have relatively standardized layouts. If there are three prongs, the two opposite are the supply lines and the third is neutral or ground. If there are four prongs, the center prongs are usually ground and neutral. The flat or angle prongs are supply lines and the round prong is protective ground. On a 110 volt plug, the smaller flat blade is the hot side.
Color coding is normally used for standardization and safety. White is neutral, black and red are hot. Green is for the protective ground. Looking at the terminals in plugs and receptacles, copper color is for hot leads, silver for neutral, and green for protective ground.
Controlling The Heat
There are three basic ways that are used to control the electricity and vary the amount of heat produced by a heating element. The most common in today's kilns is to apply pulses of full voltage and vary the length of time of those pulses. Most new kilns use this method by using a standard oven-heat switch for each element or element pair.
These switches simply pop on and off for differing lengths of time at each setting. For example, if the switch is on for ten seconds and off for ten seconds the element is putting out approximately 50% of its heating capacity. At low, the switch may be on for ten seconds, and off for forty, so the element is then putting out about 20% of what it will put out at high. (I say about because the resistance will vary with the temperature of the wire, and with changes in resistance the amount of current flow will also change.)
Another method of control is to supply full operating voltage to each element to produce full (high) output of heat or to re-connect the elements in series with one another so that each element gets less of the original voltage to produce a low output. This is how the burners on an electric stove are wired and why they will glow red hot on one setting and not on another. If, for example,
the end of one element is connected to the beginning of the next element, the amount of heat produced will be lowered by half.
The principal advantage of this method-fewer switches that last longer-is offset by more complicated wiring and fewer steps for heat control. In addition, kilns using this method may also use four prong connectors to be able to access both 220 or 110 volts for heat control. This, in turn, increases the cost of the supply wiring. (Worse yet, general unfamilarity with this type of connector can lead to dangerously faulty wiring where two supply wires instead of three are used and the lightweight protective ground wire is seriously overloaded when the kiln is on the lower settings.)
The final and seldom-used method is to use a variable-voltage supply
to control the heat. The problems of cost, size and efficiency of voltage
control devices with high-current loads limit their use in the larger electric
kilns. Furthermore, there are increased heat and electricity losses
and an increased complexity level that demands a specialist for any maintenance.
The clue to the method that your kiln uses is in the switch. If the switch functions as an "infinitely" variable switch, it probably uses the pulse method, and although there may be more wires and switches, the wiring is straightforward and simple. Two supply wires run from the source of power to each switch (Figure 3), and two additional wires run to opposite ends of each element or pair.
If the switch has only two to four settings and "snaps" from one to the next so that intermediate settings are impossible, then it is the second of the two methods. If your kiln is this type, it is essential that the wiring be duplicated exactly as the original. Because of different switch configurations and different possibilities for control, it is impossible to generalize the wiring here.
If the controls are housed in a box separate from the kiln, variable-voltage control may be used. Although "ramping" controllers are also housed externally, most of these use the pulse method described above.
As with any electric project, the work should not be reconnected to
the power supply until a qualified electrician has checked the work for
the proper circuitry and routing of wires. Since most electricians have
never seen a kiln, it is a good idea to take detailed photographs as you
remove the old elements connecting wires and switches.